Devices and systems using acoustic energy, particularly within the ultrasonic range, i.e., acoustic waves with a frequency greater than about twenty kilohertz (20 kHz), and more typically between fifty kiloHertz and ten MegaHertz (0.05-10 MHz), have been used to diagnose and treat patients. Ultrasonic energy may be employed to obtain images of a patient during a diagnostic or therapeutic procedure. In addition, ultrasound systems have been used for treating tissue, e.g., by directing acoustic energy towards a target tissue region within a patient, such as a cancerous or benign tumor, to coagulate, necrose, generate mechanical damage (by cavitation) or otherwise heat the tissue region. For example, one or more piezoelectric transducers may be disposed adjacent a patient's body and used to deliver high intensity acoustic waves, such as ultrasonic waves, at an internal tissue region of a patient to treat the tissue region. An exemplary focused ultrasound system is disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al. The acoustic energy emitted from such a system may be focused at a desired focal zone to deliver thermal energy to the target tissue region.
Focused ultrasound procedures may allow a patient to be treated while avoiding invasive surgery. For example, a focused ultrasound system that includes a single concave transducer has been used to treat breast, uterine and other tumors. Such transducer transmits an acoustic beam, which converges into a focus in target tissue to treat tissue. However, the acoustic beam may transverse through an organ, such as a breast nipple, or other sensitive areas, either before the beam converges into the focus (i.e., in a near field) or beyond the target tissue (i.e., in a far field). These areas have a high absorption coefficient compared to regular tissue, thereby risking damage to non targeted tissue at the near field and/or the far field. Also, in some cases, the acoustic beam may impinge on a tissue (e.g., bone tissue) that would not allow the beam to pass through by reflecting and/or absorbing most of the impinging energy. As a result, the acoustic beam may not reach the target tissue, and may generate undesired heating at the tissue surface that is blocking or interfering the acoustic beam. In some cases, the heating of bone tissue may also heat, and adversely affect, a nerve that is adjacent the bone tissue. A similar situation could happen with volumes in the body that are filled with air acting as total reflector for acoustic beam, thereby blocking the beam from propagating to the target tissue region.
A transducer can sometimes be positioned in a way so that the acoustic beam can reach target tissue, while reducing a risk of passing through tissues that have a high risk for damage. However, positioning the transducer in a way that will avoid tissue damage at tissue desired to be protected may complicate treatment planning and treatment procedure. Further, positioning the transducer to treat target tissue, while avoiding passage of the acoustic wave energy through tissue desired to be protected may not be practical or possible in some cases. For example, when treating a liver or a kidney, it may be necessary to deliver acoustic energy between the ribs. In such cases, moving the transducer away from the ribs, e.g., and positioning it below the ribs, may render the treatment ineffective.